The system of percussion welding is known in the art and generally operates as follows: first and second parts to be welded are clamped on top and bottom electrodes, one of the parts having a small projection called an arc starter; the parts are brought together with a very light pressure such that the arc starter of one part touches the other part; a large current is then passed through the arc starter; the arc starter cannot pass this current so it vaporizes (explodes) and an arc passes over the surfaces of both parts heating both surfaces; a follow-up force pushes the two hot surfaces together and the parts are welded.
One of the beneficial uses for percussion welding is in the attachment of electrical contacts to terminal members, especially in the attachment of so-called "non-weldable" contact material such as silver cadmium oxide (AgCdO). In electrical switching applications, it is desirable to use contacts which will not weld together and prevent opening of the circuit. On the other hand, however, such a "non-weldable" contact must somehow be attached to a terminal in order to make circuit connection. Such attachment is possible by percussion welding the contact to the terminal, whereas other types of welding, such as stud welding, induction welding, and resistance welding have been found unsuitable for such use.
The following table compares typical parameters involved in stud welding, induction welding, resistance welding, and percussion welding.
______________________________________ Stud Induction Resistance Percussion ______________________________________ part porojectons lg. cone none beveled arc starter edge current type D.C. A.C. A.C. 1/2 cycle high 60 cyc/sec A.C. frequency current size 200 Amp 100 Amp 20,000 Amp 50,000 & Amp current time 15 Sec. 3 Sec. 3 Cyc. 3 mili sec. 49 mili sec. pressure 400 lbs/ 700 lbs/ 300 lbs/ 100 lbs/ sq in sq in sq in sq in* pressure time 2 Sec. 50 mili 20 cyc 50 mili sec. sec. 333 mili sec. arc Yes No No Yes voltage 32V 3V 4V 24 + V suitable for No No No Yes welding AgCdO ______________________________________ *with follow-up force (e.g. magnetically induced) of 1000 lbs/sq in
Percussion welders, however, are subject to certain disadvantages. The explosion of the arc starter is extremely loud and soundproof boxes must be used or ear protection devices must be worn. For example, sound level measurements on a typical percussion welder in open air were recorded at about 127 decibels (dB). Occupational Safety and Health Administration (OSHA) Health Standard 1910.95 requires that exposure to impulse or impact noise not exceed 140 dB peak sound pressure level, and new standards proposed by OSHA include the following criteria for impact or impulse noise: 140 dB for not more than 100 impulses per day; 130 dB for not more than 1000 impulses per day; 120 dB for not more than 10,000 impulses per day.
Another disadvantage is that excess flash or molten material comes out the sides when the arc goes beyond the edges of the parts. In order to assure that the entire facing surface of each part is welded to the entire facing surface of the other part, the arc should spread to or just beyond the edges of the parts. However it is difficult to accurately control how far the arc spreads, and thus to ensure complete surface coverage, the arc is spread beyond the edges of the parts. The excess flash beyond the edges of the parts sticks to the tooling and equipment, thus requiring frequent cleaning, and necessitating intermittent interruption of an automated line. Also, the flash expulsion material is usually lost as scrap.
A further disadvantage is that gaseous vapors must be vented, requiring, for example, the provision of an exhaust fan system.
A still further disadvantage is the burn-off of part material due to arc-in, which can be financially significant especially when precious metal contacts are being welded.
While prior percussion welders have been useful for their intended purposes, the present invention relates to improvements thereover and alleviates the above-noted and other disadvantages.